Loss of caller display

Hi yesterday I had BT infinity installed really pleased as my tesconet broadband was painfully slow.Unfortunately since the installation my cordless phone no longer shows incoming telephone numbers ,display says unknown ,even our own alternative numbers that are in the phones directory.I have called 150 and spoke to a very polite gentleman but I don't think he really understood the problem even tho I did repeat the situation .He has just offered to send me a caller display unit.thanks in advance if anyone can help